Certain 2-aryl-1,3-cyclohexanediones and their esters are known to be extremely active, biological compounds. U.S. Pat. Nos. 4,175,135 and 4,256,657 and copending application U.S. Ser. No. 781,781 filed Mar. 28, 1977, U.S. Pat. No. 4,422,870 all of which are herein incorporated by reference, teach the usefulness of these compounds as herbicidal and miticidal agents and as agents for orally controlling acarina ectoparasites on warm blooded animals.
3-Alkyl-substituted glutaric acids are important intermediates in the manufacture of the 2-aryl-1,3-cyclohexanediones.
A number of synthetic routes for the preparation of the 3-alkyl-substituted glutaric acids; specifically the 3,3-dimethyl-glutaric acid have been described in the prior art.
The Guareschi synthesis involves an initial reaction of ethyl cyanoacetate with acetone to give dicyanoglutarimide which upon subsequent hydrolysis with dilute sulfuric acid gives 3,3-dimethyl-glutaric acid in an 55-60% yield. This process, however, uses ethyl cyanoacetate, a relatively expensive chemical; generates large quantities of ammonium sulfate; and requires long reaction times.
A second method involves the oxidation of 5,5-dimethyl-1,3-cyclohexanedione with sodium hypochlorite to produce the 3,3-dimethyl-glutaric acid. Although good yields can be realized via this method, it suffers the major disadvantages relating to the disposal of the chloroform and the copious quantities of inorganic salts generated by the reaction.
A third method uses relatively inexpensive isophorone as a starting material; generates 3,3-dimethyl-5-ketohexanoic acid from the isophorone by ozonolysis; and oxidizes the acid with sodium hypochlorite to produce the 3,3-dimethyl-glutaric acid. This method also suffers from the disposal problems attendant with the afore-described second method.
The prior art has also disclosed two processes for the oxidation of 5,5-dimethyl-1,3-cyclohexanedione.
R. H. Young, Chem. Comm; 704 (1970) reported that the photo-oxidation of 5,5-dimethyl-1,3-cyclohexanedione yielded 3,3-dimethyl-glutaric acid and Beutel, J. Am. Chem. Soc. Vol. 93, pg 2615 (1971) reported that the primary product of a radical initiator catalyzed reaction of 5,5-dimethyl-1,3-cyclohexanedione and oxygen was "a thermally unstable species which was found to decompose to 3,3-dimethyl-glutaric acid". Neither the Young nor the Beutel reaction methods are desirable for commercial operations.